Arc deposition of platinum-carbon electrically resistive films



United States Patent 3,428,541 ARC DEPOSITION OF PLATINUM-CARBON ELECTRICALLY RESISTIVE FILMS John C. Froemel, Verona, and Meyer Sapolf, West Orange, N.J., assignors to Victory Engineering Corporation, Springfield, N.J., a corporation of Delaware No Drawing. Filed June 3, 1966, Ser. No. 555,006 US. Cl. 204192 6 Claims Int. Cl. C22c /00 This invention relates to resistive films for use in electronic circuits and especially in miniature or micropower circuitry. In particular, this invention relates to an improved electrically resistive film characterized by an extremely low temperature coefficient of resistivity.

Resistive films for the purpose mentioned are not in themselves new, a number of types of such film having been heretofore proposed. While such films as have been available are useful and have gained considerable acceptance, they suffer from certain disadvantages which until now have tended to impose limitations on their usefulness.

One of the major drawbacks associated with existing resistive materials is the difiiculty of obtaining films which simultaneously provide both high resistivity and low temperature coefiicient of resistivity. Resistive films with temperature coefiicients as low as about 5 parts per million per degree centigrade (p.p.m./ C.) have been produced, with reasonable yield, by evaporating a modified nickel-chrome material onto a suitable substrate material. By painstaking control of the purity of the starting materials and of the evaporation process, it is possible to obtain films which exhibit a distribution of temperature coeflicient (TC) centered at approximately 50 p.p.m./ C. (All references herein to temperature coefficient, unless otherwise stated, refer to the temperature coefficient of resistivity.) By careful selection among the films so produced, it is possible to provide films having TCs as low as about 5 p.p.m./ C. The remainder having higher TCs, must be used in less critical applications, reworked, or discarded. The desired films, therefore, tend to be extremely costly.

Moreover, films produced in the manner just described are characterized by relatively low surface resistivities. The surface resistivity may be increased to some extent by decreasing the thickness of the film. This approach is limited, however, by the fact that when the film thickness is reduced sufliciently to provide the desired high resistivity, the film becomes discontinuous, forming islands which are in only partial contact with one another, and this results in an increase in the TC.

Other films have been proposed in the literature, having high surface resistivities, and made of different materials. In general, however, these have high TC values, for example of the order of 200-500 p.p.m./ C.

It is not always essential, and in some cases not even desirable that the TC be close to zero. For some applications it may be desirable to have the resistivity vary with temperature in a predetermined manner. For example, it may be desired to provide an appreciable TC, either positive or negative, in the film, so as to balance and compensate for temperature-induced variations of other components of the circuit. Control of the TC to any predetermined value other than zero has been until now, for all practical purposes as difficult as holding it to a predetermined value close to zero.

An object of this invention, therefore is to provide an improved electrically resistive film.

Another object is to provide a film combining the properties of high resistivity and low temperature coefficient of resistivity.

Another object is to provide a method of preparing a film of resistive material, whereby the temperature coelficient of resistivity may be controlled to a prede- 'ice termined value of temperature coeflicient over a relatively Wide range of values between a high negative value and a high positive value.

Another object is to provide a method of preparing a resistive film, whereby the surface resistivity of the film can be predetermined to a desired value ranging from moderately high to very high as compared to resistive films heretofore available, while still retaining the desired TC value.

A feature of the invention is the use of a platinumgroup metal in combination with carbon as a resistive filmforming material.

Another feature is the use of a lanthanide rare-earth metal as an additional component of the film-forming material.

Still another feature of the invention is the use of the arc-sputtering technique as a means of depositing a film of the film-forming materials on a suitable substrate.

Other objects, features and advantages will become apparent from the following more complete description and claims.

In one particularly desirable embodiment, this invention contemplates an electrically resistive film comprising a solid solution of carbon and a platinum-group metal.

In another especially desirable embodiment, this invention contemplaten a method for preparing an electrically-resistive film, which comprises in combination the steps of providing an electrode comprising carbon and a platinum-group unetal in intimate admixture, striking an are between said electrode and a counter-electrode, thereby producing a vaporized mixture of said carbon and said platinum-group metal, and condensing said vaporized mixture on a substrate to form a film comprising said carbon and said platinum-group metal.

Both carbon and platinum have been suggested in the past for use as electrically resistive materials. However, it has never to our knowledge been suggested that carbon and platinum can be successfully combined to provide a. composite material having a TC which may be predetermined to be negative, positive, or substantially zero, depending on the relative proportions of carbon and platinum employed.

Other platinum-group metals which are soluble in carbon may be employed in place of platinum. However, platinum has proven to be particularly adapted to use in the practice of the invention, and is cheaper and more readily available than most of the other platinum-group metals, and is therefore preferred. For this reason, the invention is described herein with particular reference to the use of platinum itself as the platinum-group metal. It is nevertheless to be understood that other platinumgroup metals may be substituted therefor, if so desired. The manipulation techniques involved in using platinumgroup metals other than platinum are substantially identical to those employed when platinum itself is used as the platinum-group metal, and for this reason are not separately described nor exemplified in detail.

Platinum (as well as some other platinum-group metals) is completely soluble with carbon. This fact, together with its ability to be vaporized from the electrode during arc sputtering without reacting to form oxides or other compounds, is important to the success of the invention. By selecting the proper carbon-platinum ratio, one may obtain a composition having a TC very close to zero over a considerable temperature range with excellent tracking.

The use of arc sputtering as a technique for depositing films of high-temperature stable materials as films on suitable substrates is not in itself new, and it is carried out in conventional fashion in the practice of the present invention and therefore need not be described in full detail. It has not, however, to the best of our knowledge, been previously suggested as a means for depositing a thin film of electrically resistive material on a suitable substrate, to form an electrically-resistive film useful as an electrical circuit element, nor has it been suggested to use this technique to deposit a homogeneous composite film of carbon and a platinum-group metal on a substrate for this purpose.

Depending on the temperature coefficient and resistivity desired, satisfactory results may be obtained with percentages of platinum ranging from to 90%, the balance being mainly carbon, together with impurities and minor percentages of additives. When such a composition is formed into an electrode and arc sputtered, it has been found to exhibit an extremely fine grain growth as compared to other evaporated or sputtered materials. Consequently, when it deposits on the substrate, it does so not in the form of relatively large islands separated by open spaces, but rather 'as a fine homogeneous film of particles so fine that they are well below the limits of resolution of standard electron microscopes.

During the arc sputtering operation, the thickness of the deposited film may be monitored as follows: A drop of high vacuum oil is placed on the middle of a glazed porcelain slide, and the slide is placed in the vacuum chamber adjacent to the substrate on which the film is to be deposited. As the sputtering operation proceeds, material from the cathode is deposited on the porcelain at substantially the same rate as it is deposited on the substrate. It is not deposited on the surface of the oil,

however, or more accurately it is deposited on the oil I but is immediately dispersed in the liquid, so that no permanent deposit is formed on the surface of the oil drop. As the sputtering operation continues, the oil remains substantially unchanged in apearance, while a visible deposit gradually builds up on the adjacent surface of the porcelain. When the deposit on the porcelain is sufficiently heavy to be visible as a dark brown color- 'ation, the desired film thickness, within tolerable limits, has been deposited.

Of the above-mentioned electrode materials, carbon has a negative TC and platinum a positive TC. By proper selection of the relative proportions of carbon and platinum, is is possible to provide a composition having a TC anywhere between that of pure car-hon and that of pure platinum including a composition having a TC substantially zeroi.e. one of which the specific resistivity is substantially invariant within reasonable temperature limits. The TC of pure carbon is approximately 500 p.p.m./ C. and that of platinum is approximately +3000-4000 p.p.m./ C. (measured at temperatures in the vicinity of 25 C.). A composition having a TC of substantially zero can be made by preparing, in known manner, a solid solution of- 81% carbon and 19% platinum. The corresponding percentages for solid solution of carbon with other platinum-group metals, as well as the percentage proportions needed to provide a composition having any other given TC between that of pure carbon and that of the platinum-group metal, may be readily determined by routine experimentation.

Another important property of resistive films, aside from the temperature coefficient of resistivity, or TC, is the absolute value of the resistivity. According to the present invention, the absolute resistivity may be varied over a considerable range by the introduction of a lanthanide rare earth metal, preferably hafnium or neodynium, especially the former. Using additions of hafnium, for example, a composition comprising carbon and platinum alone, in relative proportions of 81% carbon, 19% platinum (all percentages herein being by weight, unless otherwise noted), will have a surface resistivity of approximately -175,000 ohms per square, and a TC .of substantially zero. A similar composition, but also containing 0.1% hafnium, also exhibits a TC of substantially zero, but the absolute value of the surface resistivity, measured at a temperature between, approxi- 4 mately 25 C. and 25 C., is of the order of 75,000 oms per square.

The film material according to the invention may be prepared and deposited on a suitable substrate as follows: A material composed of platinum 81%, hafnium 0.1%, balance canbon and incidental impurities is powdered and mixed, and then combined with a binder such as Lucite or polyvinyl butyral. The amount of binder is not critical, so long as there is enough thereof to consolidate the powdered material to a coherent mass suitable for use as an electrode, but not so much as to dilute unduly the active materials, or unduly increase the electrical resistance of the electrode formed therefrom. In general, an amount of binder material equal to approximately 5% of the wet weight of the combined carbon, platinum. (or other platinum-group metal) and hafnium (or other rare earth) has been found to form a satisfactory electrode.

The composition of platinum-group metal, rare-earth metal and binder ispressed into a rod approximately /s-inch in diameter and two inches long. The rod is then inserted into a holder and the tip thereof is necked down to a conical or similar point approximately i -inch in diameter by Mt-inch long. This forms one electrode, while a similar rod, which is not necked down, serves as the other electrode. The necked-down rod is then brought into contact with the counter-electrodei.e. the other rod, and a suitable A.C. voltage is applied. A suitable A.C. potential 'voltage for the purpose is a 60-cycle, 110-volt potential, but this may be varied over rather wide limits, as will be apparent to those skilled in the art. The are is struck under a vacuum, preferably, of at least about 10 torr. When the necked-down portion of the rod is consumed to the point Where the tip diameter has increased to about As-inch, the process is halted, and the rod is re-dressed for further use.

To further illustrate the nature of this invention and the manner of practising the same, the following example is presented.

EXAMPLE Platinum, carbon and hafnium were mixed in proportions to form a powder mixture having the following percentage composition:

Percent (wt.)

Carbon 81.0

Hafnium 0.1

Platinum 18.9

The powdered composition, after thorough blending, was mixed with powdered polymethyl methacrylate (Lucite) in proportions sufiicient for the Lucite to serve as a binder, namely approximately 5 parts of Lucite for each 100 parts by weight of the powdered mixture.

The resulting mixture was then pressed in a hydraulic press, under a pressure of 4,000 psi. and at a temperature of 25 C., to the form of a rod approximately /s-inch in diameter and 2 inches long. The rod was necked down as above described, and used as one electrode in a conventional arc-sputtering apparatus, the other electrode being a rod of the same composition but not necked down.

A substrate to be coated was placed in the vacuum chamber of the arc-sputtering apparatus, alongside a porcelain slide having thereon a drop of high-vacuum oil. The chamber was then evacuated to a pressure of 10-" torr, and an arc was struck between the electrodes, using an applied voltage of up to volts, at an A.C. frequency of 60 cycles per second.

The are was maintained for approximately 3 minutes, until the area of the porcelain slide surrounding the oil showed a visible dark-thrown deposit.

The arc was then extinguished, the vacuum released, and the coated substrate was removed from the vacuum chamber.

The coating on the substrate was found to have a surface resistivity of 75,000 ohms per square at 25 C.

Using a Wheatstone bridge facility which had been calibrated and found to have a maximum uncertainty of 0.1%, the TC between 25 C. and 125 C. was found to be zero. Between 25 C. and 25 C., the TC was between zero and -2.4 p.p.m./ C., and between 25 C. and 40 C. it was found to be negative and less than 3 p.p.m./ C. in magnitude.

Similar experiments, carried out in the same general manner, showed that the temperature coefficient, or TC,

could be made more negative by increasing the ratio of 1 of the surface resistivity at any given temperature, and

for a given ratio of carbon to platinum, can be decreased by increasing the amount of hafnium.

While this invention has been described by way of certain preferred embodiments and illustrated by way of certain examples, these are illustrative only, as many alternatives and equivalents will readily occur to those skilled in the art, without departing from the spirit or proper scope of the invention. The invention is therefore not to be construed as limited, except as set forth in the appended claims.

In some versions of the periodic classification of elements, hafnium is not considered to be one of the lanthanide rare-earth metals, but rather the next adjacent member to the last of the rare earths, namely lutecium (atomic N0. 71), hafnium having the atomic No. 72. For purposes of the present disclosure, unless the contrary appears, hafnium is contemplated as being within the group of lanthanide rare-earth metals.

an electrode comprising carbon and a platinum-group metal in intimate admixture, striking an arc between said electrode and a counter-electrode, thereby producing a vaporized mixture of said carbon and said platinumgroup metal, and condensing said vaporized mixture on a substrate to form a film comprising said carbon and said platinum-group metal.

2. A method according to claim 1, wherein said plati num-group metal is platinum.

3. A method according to claim 1, wherein said carbon and said platinum-group metal are present in said electrode in relative proportions of the order of 81% carbon, 19% platinum-group metal, said percentages being by weight based on the combined weight of said carbon and said platinum-group metal.

4. A method according to claim 1, wherein said elec- -I. trode further comprises a lanthanide rare-earth metal in amount of the order of 0.1%, based on the combined weight of said canbon, platinum-group metal and rareearth metal.

5. A method according to claim 4, wherein said lanthanide rare-earth metal is hafnium.

6. A method according to claim 4, wherein said lanthanide rare-earth metal is neodymium.

References Cited UNITED STATES PATENTS 1,862,422 6/ 1932 Oswald 252-503 2,140,881 12/1939 Parisot 252503 FOREIGN PATENTS 830,628 3/ 1960 Great Britain. 542,599 1/1942 Great Britain. 892,889 4/ 1962 Great Britain.

R. K. M=IHALEK, Primary Examiner.

US. Cl. X.R. 

1. A METHOD FOR PREPARING AN ELECTRICALLY-RESISTIVE FILM, WHICH COMPRISES IN COMBINATION THE STEPS OF PROVIDING AN ELECTRODE COMPRISING CARBON AND A PLATINUM-GROUP METAL IN INITMATE ADMIXTURE, STRIKING AN ARC BETWEEN SAID ELETCTODE AND A COUNTER-ELECTRODE, THEREBY PRODUCING A VAPORIZED MIXTURE OF SAID CARBON AND SAID PLATINUMGROUP METAL, AND CONDENSING SAID VAPORIZED MIXTURE ON A SUBSTRATE TO FORM A FILM COMPRISING SAID CARBON AND SAID PLATINUM-GROUP METAL. 